This invention relates to magnetic bubble domain propagation circuits and in particular to a propagation circuit which employs an applied rotating magnetic field plus the effect of a gradient in the spacing between the bubble material and a magnetic film for the propagation and manipulation of bubble domains.
Memory storage in conventional magnetic bubble devices is usually accomplished by the presence or absence of bubble domains propagated and manipulated on a chip by the use of an overlay magnetic (Permalloy) circuit in conjunction with an in-plane rotating magnetic field in the presence of a bias magnetic field.
The first circuit employed for bubble propagation is the wellknown T-I bar pattern such as described in "Magnetic Bubble Technology: Integrated -- Circuit Magnetics for Digital Storage and Processing" edited by Hsu Chang, IEEE Press 1975, Library of Congress Catalog #73-87653, Page 20, paragraph entitled, "Field Access Devices -- Rotating Planar Field." A subsequent pattern employed is the Y-bar propagate element as described at page 24 of this same book.
However, the minimum feature size, that is, the smallest dimension in the pattern, which in these prior art patterns is the gap between the Y or T and I bars, is approximately 1/3 to 1/4 of the bubble diameter. Since the resolution limit of photolithography used in the process of forming the T-I and Y bar overlays is somewhere between 1 and 1.5 microns, it is evident that the smallest bubble diameter that can be used with these patterns is approximately 4.0 microns. This size is also a limitation in the storage density of the memory.
Recently, a new family of bubble propagation patterns, usually referred to as "gap tolerant" or sometimes called "half disc" or "wide gap" patterns, were described in the Intermag -- MMM conference, Pittsburgh, Pa., June 1976, by Bell Laboratories, Rockwell International and Texas Instruments employees. These gas tolerant patterns have the main advantage in that their minimum feature size, (again in this case the gap between the propagate elements), is approximately half of the bubble diameter which for a selected bubble size, for example, 4.0 microns, leads to less stringent photolithographic requirements. Conversely, if one wishes to maintain stringent photolithographic requirements, then smaller bubble sizes could be tolerated. Thus, propagation circuits for two micron bubble diameter devices using wide gap patterns can be processed by means of photolithographic methods with an improvement in memory storage density by a factor of around 3 over the conventional T-I or Y bar circuit overlays, mentioned above.
Thus, it can be seen that although the memory devices have been improved by the gap tolerant patterns, it is the minimum feature size in magnetic bubble propagation circuitry (which thus far in the prior art is the gaps between elements) which limits the size of the domains and thus the storage density of the memory devices.
Accordingly, it is apparent that if the gaps between the elements can be eliminated, then the minimum feature size can be greatly increased. In such a case then, the feature size would not be the gaps but would be the width of the element itself --approximately equal to the bubble diameter -- and thus the storage density would be greatly increased. However, elimination of the gaps alone would not suffice since directionality of propagation is imparted to the conventional T-I and Y bars and the gap tolerant circuits by means of the gaps, which gaps introduce asymmetry into the pattern. In other words, according to the prior art: no gaps, no directionality since it is the gaps that permit the formation of reversible roles in the propagate elements in response to the applied rotational magnetic field to attract the domains.
Considering the prior art further, in the U.S. Pat. No. 3,927,398 issued to Magid Y. Dimyan on Dec. 16, 1975 and entitled, "Magnetic Bubble Propagation Circuit," it was shown that a translation force acting on magnetic bubble domains can also be produced by means of gradient in spacing between bubble material and the propagate element. As described in this patent, an overlay of bars of uniform thickness of magnetic material were spaced from a film of bubble material with one end of the bar having a greater spacing than the other so as to form a gradient between the bubble material and the ends of each bar in the direction of a propagation. With this pattern, a periodic monopolar magnetic field applied in the plane of the bubble material and parallel to the propagation path, magnetized the bars causing the bubble to move from one bar to an adjacent bar across a gap between the bars. When the bars were demagnetized, i.e., the magnetic field ceased (being periodic), the bubble moved from the high end of the bar to the lower end. It is to be noted, however, that although the patented invention relied on the spacing gradient to move the bubble from one end of the bar to the other, a gap between the bars was still required to obtain an overall directionality through the device. Thus, feature size was still the gap which limited the size of the domains and the storage density of the memory device even though movement of the bubble domain was accomplished without an aplied magnetic field at certain times, albeit only within the bars themselves. This patented invention did, however, eliminate the need of a rotating magnetic field used prior thereto in the prior art devices.